Method for constructing a dipole antenna

ABSTRACT

A method of fabricating a microwave antenna assembly is disclosed. The fabrication method includes providing a proximal portion having an inner conductor and an outer conductor, the inner conductor extending at least partially therein. The method further includes providing a distal portion disposed distally of the proximal portion, with the inner conductor extending at least partially therein. A high strength material may be injected from an inflow slot to an outflow slot of the distal portion such that the material is disposed in-between the inner conductor and a ceramic layer. The material bonds the distal portion and the ceramic layer to the proximal portion while providing mechanical strength to the distal portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/556,238, filed Sep. 9, 2009, now U.S. Pat. No. 8,069,533, issued onDec. 6, 2011 and entitled “Method for Constructing a Dipole Antenna,”the contents of which are hereby incorporated by reference herein intheir entirety.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave applicators usedin tissue ablation procedures. More particularly, the present disclosureis directed to a microwave applicator having either a liquid or solidloaded tip dipole antenna.

2. Background of Related Art

Treatment of certain diseases requires destruction of malignant tissuegrowths (e.g., tumors). It is known that tumor cells denature atelevated temperatures that are slightly lower than temperaturesinjurious to surrounding healthy cells. Therefore, known treatmentmethods, such as hyperthermia therapy, heat tumor cells to temperaturesabove 41° C., while maintaining adjacent healthy cells at lowertemperatures to avoid irreversible cell damage. Such methods involveapplying electromagnetic radiation to heat tissue and include ablationand coagulation of tissue. In particular, microwave energy is used tocoagulate and/or ablate tissue to denature or kill the cancerous cells.

Microwave energy is applied via microwave ablation antennas thatpenetrate tissue to reach tumors. There are several types of microwaveantennas, such as monopole and dipole. In monopole and dipole antennas,microwave energy radiates perpendicularly from the axis of theconductor. A monopole antenna includes a single, elongated microwaveconductor. Dipole antennas may have a coaxial construction including aninner conductor and an outer conductor separated by a dielectricportion. More specifically, dipole microwave antennas may have a long,thin inner conductor that extends along a longitudinal axis of theantenna and is surrounded by an outer conductor. In certain variations,a portion or portions of the outer conductor may be selectively removedto provide for more effective outward radiation of energy. This type ofmicrowave antenna construction is typically referred to as a “leakywaveguide” or “leaky coaxial” antenna.

Conventional microwave antennas typically has a long, thin innerconductor which extends along the axis of the probe and is surrounded bya dielectric material and is further surrounded by an outer conductoraround the dielectric material such that the outer conductor alsoextends along the axis of the probe. In another variation of the probewhich provides for effective outward radiation of energy or heating, aportion or portions of the outer conductor can be selectively removed.This type of construction is typically referred to as a “leakywaveguide” or “leaky coaxial” antenna. Another variation on themicrowave probe involves having the tip formed in a uniform spiralpattern, such as a helix, to provide the necessary configuration foreffective radiation. This variation can be used to direct energy in aparticular direction, e.g., perpendicular to the axis, in a forwarddirection (i.e., towards the distal end of the antenna), or acombination thereof.

Invasive procedures and devices have been developed in which a microwaveantenna probe may be either inserted directly into a point of treatmentvia a normal body orifice or percutaneously inserted. Such invasiveprocedures and devices potentially provide better temperature control ofthe tissue being treated. Because of the small difference between thetemperature required for denaturing malignant cells and the temperatureinjurious to healthy cells, a known heating pattern and predictabletemperature control is important so that heating is confined to thetissue to be treated. For instance, hyperthermia treatment at thethreshold temperature of about 41.5° C. generally has little effect onmost malignant growths of cells. However, at slightly elevatedtemperatures above the approximate range of 43° C. to 45° C., thermaldamage to most types of normal cells is routinely observed; accordingly,great care must be taken not to exceed these temperatures in healthytissue.

However, many types of malignancies are difficult to reach and treatusing non-invasive techniques or by using invasive antenna probesdesigned to be inserted into a normal body orifice, i.e., a body openingwhich is easily accessible. These types of conventional probes may bemore flexible and may also avoid the need to separately sterilize theprobe; however, they are structurally weak and typically require the useof an introducer or catheter to gain access to within the body. Further,the manufacturing techniques for the conventional probe tend to becumbersome, time consuming, and prohibitively expensive. Moreover, theaddition of introducers and catheters necessarily increase the diameterof the incision or access opening into the body thereby making the useof such probes more invasive and further increasing the probability ofany complications that may arise.

SUMMARY

A method of fabricating a microwave antenna assembly, which isstructurally robust enough for unaided direct insertion into tissue isdescribed herein. The microwave antenna assembly is generally comprisedof a radiating portion which may be connected to a feedline (or shaft)which in turn may be connected by a cable to a power generating sourcesuch as a generator. The microwave assembly may be a monopole microwaveantenna assembly but is preferably a dipole assembly. The distal portionof the radiating portion preferably has a tapered end which terminatesat a tip to allow for the direct insertion into tissue with minimalresistance. The proximal portion is located proximally of the distalportion.

The adequate rigidity necessary for unaided direct insertion of theantenna assembly into tissue, e.g., percutaneously, preferably comes inpart by a variety of different methods. A method of fabricating anantenna includes providing a proximal portion having an inner conductorand an outer conductor, the inner conductor extending at least partiallytherein. The method further includes providing a distal portion disposeddistally of the proximal portion, with the inner conductor extending atleast partially therein. A high strength polyimide material may beinjected from an inflow slot to an outflow slot of the distal portionsuch that the polyimide material is disposed in-between the innerconductor and a ceramic layer. The polyimide material bonds the distalportion and the ceramic layer to the proximal portion while providingmechanical strength to the distal portion.

To further aid in strengthening the antenna assemblies the innerconductor may be affixed within the distal radiating portion in avariety of ways, for instance, welding, brazing, soldering, or throughthe use of adhesives. Forcing the inner conductor into a tensilecondition helps to force the outer diameter of the antenna into acompressive state. This bi-directional stress state in turn aids inrigidizing the antenna assembly.

To enable a compressive state to exist near the outer diameter of thedistal portion, a ceramic layer may be bonded to the polyimide material.Materials such as ceramic generally have mechanical properties wherefracturing or cracking in the material is more likely to occur undertensile loading conditions. Accordingly, placing the distal portionunder pre-stressed conditions, may aid in preventing mechanical failureof the distal portion if the antenna were to incur bending momentsduring insertion into tissue which could subject the distal portionunder tensile loads. The ceramic layer and a coolant jacket also act asa dielectric buffer for aiding in keeping the efficiency of the antennaconstant event though tissue is changing.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentdisclosure will become more apparent in light of the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic diagram of a microwave ablation system accordingto an embodiment of the present disclosure;

FIG. 2 is an isometric view of a microwave antenna assembly according tothe present disclosure;

FIG. 3 is an enlarged, cross-sectional view of a portion of themicrowave antenna assembly of FIG. 2;

FIG. 4 is an enlarged, cross-sectional view of a portion of themicrowave antenna assembly of FIG. 2;

FIG. 5 is a side view of a distal portion of a feedline of the microwaveantenna assembly of FIG. 2;

FIG. 6 is an exploded view of the microwave antenna assembly accordingto the present disclosure;

FIGS. 7A-7C are enlarged cross-sectional views of sections A-A, B-B, andC-C of the microwave antenna assembly of FIG. 4;

FIG. 8 is a schematic diagram of a mold according to the presentdisclosure; and

FIG. 9 is a side view of another tip of the microwave assembly of FIG.2.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure will be describedherein below with reference to the accompanying drawings. In thefollowing description, well-known functions or constructions are notdescribed in detail to avoid obscuring the present disclosure inunnecessary detail.

FIG. 1 shows a microwave ablation system 10 that includes a microwaveantenna assembly 12 coupled to a microwave generator 14 via a flexiblecoaxial cable 16. The microwave antenna assembly 12 may be a dipoleantenna of 1.6 cm in length. In order to ablate small tumors, microwaveantenna assembly 12 has a Short Radiating Section (SRS). The microwaveantenna assembly 12 is capable of reducing the antenna length toone-quarter of the wavelength length required, effectively using halfthe length of the half wave length dipole antenna. The generator 14 isconfigured to provide microwave energy at an operational frequency fromabout 500 MHz to about 5000 MHz.

Antenna assembly 12 is generally comprised of radiating portion 18,which may be connected by feedline 20 (or shaft) to the cable 16. Morespecifically, the antenna assembly 12 is coupled to the cable 16 througha connection hub 22. The connection hub 22 also includes an outlet fluidport 30 and an inlet fluid port 32 defined therein that are in fluidcommunication with a coolant jacket 38 and flow channel 13 (see FIG. 4).The coolant jacket 38 encloses a proximal portion 42 and the feedline 20allowing coolant fluid from the ports 30 and 32 to be supplied andcirculated around a portion of the antenna assembly 12. The ports 30 and32 also include inner lumens defined therein (not shown) that are influid communication with the flow channel 13. The ports 30 and 32 arecoupled to a supply pump 34 that is, in turn, coupled to a supply tank36. The supply tank 36 stores the coolant fluid and maintains the fluidat a predetermined temperature. In one embodiment, the supply tank 36may include a coolant unit which cools the returning liquid from theantenna assembly 12. In another embodiment, the coolant fluid may be agas and/or a mixture of fluid and gas.

Assembly 12 also includes a tip 48 having a tapered end 24 thatterminates, in one embodiment, at a pointed end 26 to allow forinsertion into tissue with minimal resistance at a distal end of theradiating portion 18. In those cases where the radiating portion 18 isinserted into a pre-existing opening, tip 48 may be rounded or flat.

FIG. 2 illustrates the radiating portion 18 of the antenna assembly 12having an unbalanced dipole antenna 40. The dipole antenna 40 includes aproximal portion 42 and a distal portion 44 interconnected by aninjection molded seal 46. The distal portion 44 and the proximal portion42 are of different, unequal lengths so that the dipole antenna 40 isunbalanced. In one embodiment, the distal portion 44 may be longer thanthe proximal portion 42. In one embodiment, in which the feedline 20 isformed from a coaxial cable, the outer conductor 56 and the innerinsulator 52 may be sliced off to reveal the inner conductor 50, asshown in FIG. 5.

The dipole antenna 40 is coupled to the feedline 20 that electricallyconnects antenna assembly 12 to the generator 14 (FIG. 1). The assembly12 includes a coolant jacket 38 coupled to a fluid seal 8 (see FIG. 4),which in turn is coupled to an injection molded seal 46. The coolantjacket 38 may be formed from a medical grade metal. The injection moldedseal 46 may be made of a high strength polyimide resin. The polyimideresin may be VESPEL® sold by DuPont of Wilmington, Del.

In one embodiment, the injection molded seal 46 is fabricated byinjecting a polyimide material into an inflow slot 9A to an outflow slot9B of the distal portion 44. As shown in FIG. 3, the injection moldedseal 46 is disposed in-between a distal radiating section 44 and theceramic layer 2. The ceramic layer may be made of alumina ceramic. Theinjection molded seal 46 bonds the distal portion 44 and the ceramiclayer 2 to the proximal portion 42 while providing mechanical strengthto the distal portion 44. As shown in FIGS. 3-4, the feedline 20includes an inner conductor 50 (e.g., wire) surrounded by an innerinsulator 52, which is then surrounded by an outer conductor 56 (e.g.,cylindrical conducting sheath). The inner and outer conductors 50, 56may be constructed of copper, gold, stainless steel or other conductivemetals with similar conductivity values. The metals may be plated withother materials, e.g., other conductive materials, to improve theirproperties, e.g., to improve conductivity or decrease energy loss, etc.In one embodiment, the inner insulator layer 52 is formed from afluoropolymer, such as tetrafluorethylene, perfluorpropylene, and thelike, and has a thickness of about 0.011-0.013 inches.

In one embodiment, the feedline 20 may be formed from a coaxialsemi-rigid or flexible cable having a wire with a 0.047″ outer diameterrated for 50 Ohms. The inner insulator 52 may have a dielectric constantfrom about 1 to 10.

Overlaying the outer conductor 56 is a flow channel 13 that cools themajority of the proximal portion 42. The flow channel 13 is in fluidcommunication with fluid ports 30, 32. A polyimide inflow sleeve 15 isdisposed in the flow channel 13 to create an inflow channel 17 a and anoutflow channel 17 b for the coolant. The abundance of cooling fluidfrom the concentric in-flow design of the polyimide inflow sleeve 15 inthe flow channel 13 acts as a lossy material to absorb the microwaveenergy as well as to cool the feedline 20 for percutaneous use.

In another embodiment, the fluid seal 8 may also be formed from solidwire machined component or a cylindrical conductor filled with solder.The fluid seal 8 is thereafter coupled to the outer conductor 56 (jointE), as shown in FIGS. 3-4. This may be accomplished by soldering thefluid seal 8 to the outer conductor 56, such as by melting the solder ofthe fluid seal 8 and inserting the outer conductor 56 therein.

The distal portion 44 includes a conductive member 45 that may be formedfrom any type of conductive material, such as metals (e.g., copper,stainless steel, tin, and various alloys thereof). The distal portion 44may have a solid structure and may be formed from solid wire (e.g., 10AWG). In another embodiment, the distal portion 44 may be formed from ahollow sleeve of an outer conductor of coaxial cable or anothercylindrical conductor. The cylindrical conductor may then be filled withsolder to convert the cylinder into a solid shaft. More specifically,the solder may be heated to a temperature sufficient to liquefy thesolder within the cylindrical conductor (e.g., 500° F.), therebycreating a solid shaft.

As shown in FIGS. 2 and 3, the distal portion 44 is coupled to the tip48, which may be formed from a variety of heat-resistant materialssuitable for penetrating tissue, such as metals (e.g., stainless steel)and various thermoplastic materials, such as poletherimide, polyimidethermoplastic resins, an example of which is Ultem® sold by GeneralElectric Co. of Fairfield, Conn. The tip 48 may be machined from variousstock rods to obtain a desired shape. The tip 48 may be attached to thedistal portion 44 using various adhesives, such as epoxy seal. If thetip 48 is metal, the tip 48 may be soldered to the distal portion 44 ormay be machined as one continuous component.

FIG. 6 is an exploded view of the microwave antenna assembly 12. Themicrowave antenna assembly 12 includes a proximal portion 42 and adistal portion 44. The proximal portion 42 may include an innerconductor 50, an inner insulator layer 52, and an outer conductor 56.The proximal portion 42 may also include a flow channel 13 definedtherein (not shown) that includes an inflow channel 17 a and an outflowchannel 17 b that are separated by a polyimide inflow tube 15. Thepolyimide inflow tube 15 (not shown) may be inserted into a pocket of afluid seal 8 to cool the proximal portion 42. The distal portion 44includes an inner conductor 50, a distal radiating section 51, aninjection molded seal 46, and a ceramic layer 2.

FIGS. 7A-7C are enlarged cross-sectional views of sections A-A, B-B, andC-C of the microwave antenna assembly of FIG. 4. FIG. 7A illustrates across section at section A-A. Section A-A illustrates from the insidetowards the outer surface, an inner conductor 50, insulator 52, outerconductor 56, flow channel 13 (specifically inflow channel 17 a),polyimide inflow tube 15, flow channel 13 (specifically outflow channel17 b), and coolant jacket 38.

FIG. 7B illustrates a cross section at section B-B. Section B-Billustrates from the inside towards the outer surface, an innerconductor 50, insulator 52, outer conductor 56, flow channel 13(specifically inflow channel 17 a), polyimide inflow tube 15, flowchannel 13 (specifically outflow channel 17 b), fluid seal 8, andcoolant jacket 38. FIG. 7C illustrates a cross section taken at sectionC-C of the distal portion. Section C-C illustrates from the insidetowards the outer surface, an inner conductor 50, insulator 52, aninjection molded seal 46, and a ceramic layer 2.

Referring back to FIGS. 2-4, the microwave antenna assembly 12 may bemanufactured in various steps. A coaxial cable that includes the innerconductor 50, insulator layer 52, and outer conductor 56 may bemanufactured and assembled as one component. The outer conductor 56 maybe soldered to the fluid seal 8, for example at joint E, to provide theelectrical joint, if needed. The coolant jacket 38 may be bonded,threaded, laser welded, soldered or crimped to the fluid seal 8 at jointD. The coolant jacket 38 and the fluid seal 8 may be assembled as onecomponent. The polyimide inflow tube 15 may be configurable to slideinto a pocket of the fluid seal 8.

The inner conductor 50 is configured to slide inside a hole of thedistal portion 44. The distal portion 44 is affixed to a distal end ofthe inner conductor 50 by laser welding, soldering or crimping at jointF. The coaxial cable, coolant jacket 38, proximal portion 42, ceramiclayer 2, and distal portion 44 are placed into an injection mold cavity.

FIG. 8 is a schematic diagram of a mold according to the presentdisclosure. The mold 53 is used to inject a high strength polyimidematerial in-between the inner conductor 50 and the ceramic layer 2. Themold 53 includes mold halves 111 a and 111 b. Mold halves 111 a and 111b include portions/cavities to receive coaxial cable, coolant jacket 38,fluid seal 8, ceramic layer 2, and trocar tip 48. Mold halves 111 a and111 b also include an inflow slot 9A and an outflow slot 9B. The moldhalves 111 a and 111 b are clamped tightly together and heated polyimideis injected into the inflow slot 9A until the heated polyimide fillsinto outflow slot 9B defined therein. The polyimide flows into a cavityto form a uniform layer of polyimide layer along the distal portion 44.The polyimide material bonds the distal portion 44 and the ceramic layerto the proximal portion while providing mechanical strength to thedistal portion.

In another embodiment, the mold 53 does not include a cavity for thetrocar tip 48. In such an embodiment, when the injection molding processis complete, the antenna assembly 12 is finished by installing thetrocar tip 48. FIG. 9 illustrates various shapes and forms of a trocartip 48 installed onto a sheath 38, namely a stainless steel tip 48 a anda dielectric tip 48 b. Both tips 48 a and 48 b include insertion bases51 a and 51 b having an external diameter that is smaller than diameterof the tips 48 a and 49 allowing for easier insertion into a sheath 38.The configuration also provides for a better seal between the tip 48 andthe sheath 38. In another embodiment, the sheath 38 and tip 48 c maybethreaded so as to attach to each other. Therefore, the tip 48 c may betightly screwed into the sheath 38.

The described embodiments of the present disclosure are intended to beillustrative rather than restrictive, and are not intended to representevery embodiment of the present disclosure. Various modifications andvariations can be made without departing from the spirit or scope of thedisclosure as set forth in the following claims both literally and inequivalents recognized in law.

What is claimed is:
 1. A method of constructing a dipole antenna,comprising the steps of: providing an upper mold half and a lower moldhalf having mating surfaces that mate together to form cavities toreceive a feedline, radiating portion, and ceramic portions therein,each of the upper and lower mold halves includes first and second slots,respectively, to receive a polyimide material; placing the feedline intothe respective cavity, the feedline including an inner conductor, anouter conductor and an inner insulator disposed therebetween; placingthe radiating portion into the respective cavity, the radiating portionincluding an unbalanced dipole antenna having a proximal portion and adistal portion of different lengths, wherein the proximal portionincludes at least a portion of the inner conductor and the innerinsulator and the distal portion includes a conductive member; placingthe ceramic portions into the respective cavity; mating the upper moldhalf and the lower mold half together; and depositing the polyimidematerial into each of the first slots through the second slots and uponcooling, the polyimide material adheres to the distal and ceramicportions to the proximal portion.
 2. The method of constructing thedipole antenna in accordance with claim 1, wherein the distal portion isaffixed to a distal end of the inner conductor by at least one of laserwelding, soldering, and crimping.
 3. The method of constructing thedipole antenna in accordance with claim 1, further comprising the stepof joining a trocar adapted to receive a distal end of the innerconductor.